We present an in-depth investigation of Au nanoparticles self-assembled on a zigzag-like TiO(x)/Pt(111) ultrathin polar film, whose structure is known in great detail. The peculiar pattern of defects (picoholes) templates a linear array of size-selected (ca. 1 nm) Au nanoparticles without disruption of the titania layer, as observed by scanning tunneling microscopy. Their structure and electronic properties have been investigated by several large-area spectroscopic tools, i.e. high-resolution core and valence level photoemission and angle-scanned and energy-scanned photoelectron diffraction. The comparison between experimental data and density functional theoretical calculations indicates that the Au atoms landing on the oxide film are rather mobile, and that the picoholes can act as effective trapping and nucleation centers for the growth of the Au nanoparticles. All the experimental results are in concord in indicating that the Au NPs are flat islands with a maximum thickness of 2-3 layers exposing the (111) surface.